Monochromatic light of wavelength 600 nm is incident from air onto a water surface. The wavelength and speed of light respectively in water will be (Given: Refractive index of water = 4/3)
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → \( 450\,\mathrm{nm};\ 2.25\times10^{8}\,\mathrm{m/s} \)
On entering water the frequency stays the same, while the speed and the wavelength are both divided by the refractive index.
\( \displaystyle v = \frac{c}{n} = \frac{3\times10^{8}}{4/3} = 2.25\times10^{8}\,\mathrm{m/s} \)
\( \displaystyle \lambda_w = \frac{\lambda}{n} = \frac{600\,\mathrm{nm}}{4/3} = 450\,\mathrm{nm} \)
Note: NTA picked this question from an NCERT (2026-27) exercise and changed the figures. Do not ignore NCERT while practising numericals.
NCERT (2026-27):
Exercises – …
